1. Field of the Invention
The present invention is related to an optical pickup which reads information written on an optical disk, and an optical disk system equipped with an optical pickup.
2. Description of the Prior Art
Because a laser diode used in an optical pickup is an open-type cavity, the laser diode forms a cavity easily even with an external reflector, and because the emitted light of the laser diode is reflected back from the outside as return light, the oscillation can become unstable when such return light is incident on the laser diode. For this reason, when there is a lot of return light, noise is created in the emitted light of the laser diode.
For example, when the emitted light from the laser diode of the optical pickup is focused on an optical disk, the light is diffracted by the pits on the optical disk and forms modulated reflected light. This reflected light passes through an objective lens of the optical pickup and optical disk are read. In prior art optical pickups, because all the reflected light from the optical disk is made to be incident on the light detector by a polarizing beam splitter and a λ/4 plate, the reflected light does not return to the laser diode. Recently, a low-cost half mirror has been used, and in this case approximately 50% of the reflected light returns to the laser diode as return light. In the case where the laser diode receives the return light from the optical disk, the laser diode oscillates in a compound cavity mode which combines the two cavities of an internal cavity formed between the front facet and the rear facet of the diode chip, and an external cavity formed between the optical disk and the front facet of the diode chip.
As a method of reducing the effect of the return light, Japanese Patent Application No. JP59-009086-B discloses a technique for reducing the effect of the return light by superimposing a high-frequency electric current on the direct electric current driving the laser diode oscillating in a single mode in order to create a multiple mode oscillation. This technique is applied to an AlGaAs high-output laser diode and an InAlGaP visible light laser diode which are difficult to oscillate in a multiple mode with only a direct electric current driving.
In prior art optical pickups, the length of the optical path between the optical disk and the front facet of the laser diode is from 50 mm to about 70 mm, and is long compared to the coherent distance of the emitted light of the laser diode. In this case, even when return light is incident on the laser diode, because an external cavity is not formed, there are no large changes to the oscillation state of the laser diode. In accordance with the miniaturization of apparatuses in recent years, the length of the optical path between the optical disk and the front facet of the laser diode in the optical pickup has been shortened to about 30 mm, but even in such multiple mode oscillating laser diodes, the effect of the return light can not be ignored.
In this connection, Japanese Patent Application No. JP05-089465-A discloses a technique for creating a multiple mode oscillation by superimposing a high-frequency electric current on the direct electric current, wherein the frequency of the high-frequency electric current is controlled in accordance with the length of the optical path between the optical disk and the laser diode so that the return light to the laser diode returns to the laser diode during the period when the laser oscillation is stopped. However, in the case of a CD-R/RW, MD and DVD, because the optical disk has a small diameter and the playback device has a small size, the length of the optical path between the optical disk and the front facet of the laser diode is from 30 mm to 50 mm. With regard to this optical path length, when the reflected return light to the laser diode is made to return to the laser diode during the period when the laser oscillation is stopped, the frequency of the high-frequency electric current changes from 2.5 GHz to 1.5 GHz. A strong shielding is required around the driving circuit to prevent this high-frequency electric current from having an effect on the other electronic circuits inside the apparatus and it is not realistic.
Further, Japanese Patent Application No. JP08-139418-A proposes a method of superimposing a high-frequency electric current having a frequency of 400 MHz or higher in accordance with the length of the optical path between the optical disk and the front facet of the laser diode. However, in optical pickups which use current CD-R/RW, MD and DVD, the laser diode has parallel capacitance resulting from the pn reverse bias barrier, and the lead wire for connecting the laser diode chip and the package has inductance. Accordingly, because the resonance frequency is determined by the parallel capacitance and the inductance, it is difficult to use this method of superimposing a high-frequency electric current having a frequency of 400 MHz or higher.
On the other hand, R. Lang reports the effect due to return light in a single mode oscillation laser diode (IEEE Journal of Quantum Electron., QE-16, p. 347, 1980). According to this report, in the case where the distance from the front facet of the laser diode to an external mirror is an integer multiple of the effective cavity length of the laser diode, the emitted light of the laser diode does not receive an effect, but in the case where the distance from the front facet of the laser diode to an external mirror is shifted away from an integer multiple of the effective cavity length of the laser diode, because the phase conditions of the internal cavity of the laser diode and the external cavity are shifted, it is clearly understood that the laser oscillation is unstable and high-frequency noise is included in the emitted light. In this regard, when the index of refraction of the laser diode is given by m, the effective cavity length is the length represented by “cavity length of laser diode×m”. This report is for the case of a single mode oscillation laser diode, and it is unclear whether or not the results can be applied to a multiple mode oscillation laser diode.
Further, in optical pickups, it is extremely difficult to measure whether the length of the optical path between the optical disk and the front facet of the laser diode is a multiple of the effective cavity length. Namely, because the index of refraction m of the laser diode changes depending on the light confining ratio of the laser diode, large errors occur when the effective cavity length is compared with about 20 times the optical path length. Further, it is difficult to accurately measure the thickness of an optical element lying between the optical disk and the front facet of the laser diode.